Nexa Resources is a large-scale, integrated and low-cost zinc producer with more than 60 years of experience in mining and metallurgy assets. The company has a proactive approach and is constantly looking for new ways to improve its process. In early 2020, the company addressed a challenge at its zinc mine in Vazante, Brazil, where the ore content has decreased from 12% to 10% over the last decade. This led Nexa to explore alternatives to increase the plant throughput with a focus on the classification stage, a crucial factor to increase grinding capacity.
Nexa Resources adopts Metso Outotec high-frequency screen in Vazante and increases grinding circuit throughput
Classification in integrated circuit joins hydrocyclone and high-frequency screen
Nexa decided to test a mixed classification system, integrating hydrocyclones and high-frequency screens in one of its grinding circuits, Circuit C, in a cyclone cluster and ball mill. Metso Outotec was chosen as supplier and evaluation partner. Circuit C reconfiguration was performed in early 2020 and included a try-and-buy approach. In other words, Nexa initially tested the equipment integration before purchasing the solution.
In February 2020, a Metso Outotec 10-deck UltraFine screen was installed and integrated with a hydrocyclone. During a 10-month trial period, the more efficient classification resulted in increased processing capacity. It enabled the Vazante unit to operate with a production capacity of 24.8 thousand tons/year of extracted ore and 1.2 thousand tons of zinc concentrate.
In addition, the change in Circuit C resulted in a 7% increase in grinding capacity, allowing not only primary ore processing, but also tailing reprocessing. The combination of a hydrocyclone with a high-frequency screen had already been successfully adopted in Circuit W. And it was also implemented in Circuit C, making the entire grinding circuit run with a mixed classification system of hydrocyclones and screens.
“As Grinding Circuit W was already operating with a high-frequency screen, we concluded that Metso Outotec UltraFine screen, with twice the capacity compared to other similar screens, could be integrated into the process smoothly,” explains Oscar Sakurada, Senior Product Engineer at Metso Outotec.
In addition, Metso Outotec screening media was installed on the high-frequency screen in both grinding Circuits C and W.
It is important to note that classification traditionally allows only for hydrocyclone or screen use. However, the experience with Circuit W had proved the advantage of the integration. The conclusion was also validated with laboratory simulations and industrial tests performed during the trial period of Circuit C.
Challenges faced together ensured the project success
The overall trial target set was that Metso Outotec screen should increase throughput by 7%, in addition to maintaining the processed material granulometric index. The industrial test was coordinated by Nexa Resources technology area, represented by its process engineer, Larissa Rodrigues Demuner. Larissa explains that the material granulometry has a direct influence on the flotation efficiency – Nexa process following the grinding stage of zinc ore concentration in Vazante.
“The granulometric target of our operation is 12% of material accumulated on the 0.150 mm mesh. It is a critical index, and we knew it would be useless to increase the production volume without maintaining this index,” says Larissa.
This condition was met and the advantages were proved at the end of the process, however, there were challenges along the way. The first was the screen chute liner. Originally made of rubber, the liner wore excessively after the first months of operation resulting in leaks and consequently reduced throughput. The solution was to adopt liners with Metso Outotec Trellex Poly-Cer technology, which includes ceramic components and that had already been adopted in several plants around the world. The new, more wear-resistant liner met the durability level expected by Nexa.
Another challenge was the screen rotation adjustment to achieve maximum capacity with high screening efficiency. To solve this, Nexa and Metso Outotec team joint action was to adopt a gradual rotation adjustment by means of a frequency inverter.
“Each type of ore has a specific screening behavior. For this reason, we always recommend running screening laboratory tests,” explains Oscar.
The third challenge involved the equipment operational availability because the ore mined in Vazante is deposited between dolomite formations (calcium magnesium carbonate embedding rocks). As the screening media apertures are quite small (0.18 mm), carbonates crystallize on the screening media surface obstructing the holes, which demands screening media weekly replacements. This routine took 12 hours a week. Although the circuit was not paralyzed (a valve system allows the operation only with the hydrocyclone), the downtime was annoying. To solve the issue, Metso Outotec and Nexa developed a new screening media attachment system that reduces the replacement time by more than 50%.
The positive result led to the definitive adoption of high frequency screens
With the adjustments made during the try-and-buy period, Nexa Resources decided upon the definitive purchase of Metso Outotec UltraFine screen.
“The technical support is a great advantage. If the process involved only the screen sale, we would not have achieved the results obtained with the combined equipment use at the plant and the joint test phase,” evaluates Larissa.
Internally, the project was a finalist in the 2020 Nexa Recognition Program.
The cooperation between Nexa and Metso Outotec continues and new ways to optimize the process are being evaluated. The next step will be a pilot test with separate processes, a modification to implement a bypass.
In addition to the classification process modification of grinding Circuit C, Metso Outotec also assisted in the repair of a ball mill at the Vazante plant. The mill was damaged, but its operation was reestablished in a record-setting time of less than 30 days. Vazante operation also demanded other interventions in recent years, among them was the replacement of post-crushing screens, in 2016, when the company adopted the self-cleaning screen technology increasing the efficiency required for the process.
Nexa’s current operation in Vazante produced approximately 370 thousand tons of zinc concentrate in 2020, fully shipped to Três Marias plant (MG), with capacity to process 3.9 thousand tons per day.
Nexa Resources is one of the largest zinc miners in the world, in addition to producing copper and lead. It has been operating for over 60 years in the mining and metallurgy segments, with operations located in Brazil and Peru and offices in Luxembourg and the United States, supplying its products to all continents in the world. Its employees work every day focused on building the mining of the future, to become increasingly sustainable, innovative and with the best safety practices and respect for people and the environment. Since 2017, its shares are listed on the New York and Toronto Stock Exchanges, with its majority shareholder being Votorantim S.A. For more information about Nexa, visit www.nexaresources.com